In forming integrated circuits, epitaxial layers are often desired in selected locations, such as active area mesas among field isolation regions, or even more particularly over defined source and drain regions. While non-epitaxial material, which can be amorphous or polycrystalline, can be selectively removed from over the field isolation regions after deposition, it is typically considered more efficient to simultaneously provide chemical vapor deposition (“CVD”) and etching chemicals, and to tune conditions to result in zero net deposition over insulating regions and net epitaxial deposition over exposed semiconductor windows. This process, known as selective epitaxial CVD, takes advantage of slow nucleation of typical semiconductor deposition processes on insulators like silicon oxide or silicon nitride. Such selective epitaxial CVD also takes advantage of the naturally greater susceptibility of amorphous and polycrystalline materials to etchants, as compared to the susceptibility of epitaxial layers.
Examples of the many situations in which selective epitaxial formation of semiconductor layers is desirable include a number of schemes for producing strain. The electrical properties of semiconductor materials, such as silicon, carbon-doped silicon, germanium, and silicon germanium alloys, are influenced by the degree to which the materials are strained. For example, semiconductor materials can exhibit enhanced electron mobility under tensile strain, which is particularly desirable for NMOS devices; and enhanced hole mobility under compressive strain, which is particularly desirable for PMOS devices. Methods of enhancing the performance of semiconductor materials are of considerable interest and have potential applications in a variety of semiconductor processing applications. Semiconductor processing is typically used in the fabrication of integrated circuits, which entails particularly stringent quality demands, as well as in a variety of other fields. For example, semiconductor processing techniques are also used in the fabrication of flat panel displays using a wide variety of technologies, as well as in the fabrication of microelectromechanical systems (“MEMS”).
A number of approaches for inducing strain in silicon- and germanium-containing materials have focused on exploiting the differences in the lattice constants between various crystalline materials. For example, the lattice constant for crystalline germanium is 5.65 Å, the lattice constant for crystalline silicon is 5.431 Å, and the lattice constant for diamond carbon is 3.567 Å. Heteroepitaxy involves depositing thin layers of a particular crystalline material onto a different crystalline material in such a way that the deposited layer adopts the lattice constant of the underlying crystal material. For example, using this approach, strained silicon germanium layers can be formed by heteroepitaxial deposition onto single crystal silicon substrates. Because the germanium atoms are slightly larger than the silicon atoms and the deposited heteroepitaxial silicon germanium is constrained to the smaller lattice constant of the silicon beneath it, the silicon germanium is compressively strained to a degree that varies as a function of the germanium content. Typically, the band gap for the silicon germanium layer decreases monotonically from 1.12 eV for pure silicon to 0.67 eV for pure germanium as the germanium content in the silicon germanium increases. In another approach, tensile strain is formed in a thin single crystalline silicon layer by heteroepitaxially depositing the silicon layer onto a relaxed silicon germanium layer. In this example, the heteroepitaxially deposited silicon is strained because its lattice constant is constrained to the larger lattice constant of the relaxed silicon germanium beneath it. A tensile strained channel typically exhibits increased electron mobility, and a compressively strained channel exhibits increased hole mobility.
In these examples, strain is introduced into single crystalline silicon-containing materials by replacing silicon atoms with other atoms in the lattice structure. This technique is typically referred to as substitutional doping. For example, substitution of germanium atoms for some of the silicon atoms in the lattice structure of single crystalline silicon produces a compressive strain in the resulting substitutionally doped single crystalline silicon material because the germanium atoms are larger than the silicon atoms that they replace. It is possible to introduce a tensile strain into single crystalline silicon by substitutional doping with carbon because carbon atoms are smaller than the silicon atoms that they replace. Additional details are provided in “Substitutional Carbon Incorporation and Electronic Characterization of Si1-yCy/Si and Si1-x-yGexCy/Si Heterojunctions” by Judy L. Hoyt, Chapter 3 in “Silicon-Germanium Carbon Alloy”, Taylor and Francis, pp. 59-89 (New York 2002), referred to herein as “the Hoyt article.” However, non-substitutional impurities will not induce strain.
Similarly, electrical dopants should also be substitutionally incorporated into epitaxial layers in order to be electrically active. Either the dopants are incorporated as deposited or the substrate should be annealed to achieve the desired level of substitutionality and dopant activation. In situ doping of either impurities for tailored lattice constant or electrical dopants is often preferred over ex situ doping followed by annealing to incorporate the dopant into the lattice structure because the annealing consumes thermal budget. However, in practice in situ substitutional doping is complicated by the tendency for the dopant to incorporate non-substitutionally during deposition, for example, by incorporating interstitially in domains or clusters within the silicon rather than by substituting for silicon atoms in the lattice structure. Non-substitutional doping complicates, for example, carbon doping of silicon, carbon doping of silicon germanium, and doping of semiconductors with electrically active dopants. As illustrated in FIG. 3.10 at page 73 of the Hoyt article, prior deposition methods have been used to make crystalline silicon having an in situ doped substitutional carbon content of up to 2.3 atomic %, which corresponds to a lattice spacing of over 5.4 Å and a tensile stress of less than 1.0 GPa.
Source and drain recesses can be filled with a silicon-containing alloy as a “stressor” to exert a compressive or tensile strain on the silicon channel between the source and drain. For example, strained epitaxial silicon germanium (“SiGe”) in source and drain recesses can exert a compressive strain on the silicon channel and enhance hole mobility. Similarly, a carbon-doped silicon (“Si:C”) epitaxial alloy under a tensile strain in source/drain recesses can introduce a tensile strain on the channel and enhance electron mobility. In general, the strain on the channel is related to the concentration of the impurity, such as C or Ge. In other words, the higher the Ge or C content, the higher the strain produced.